1. Field of the Invention
The present invention relates to a toner for use in developing electrostatic latent images formed by a method such as electrophotography, electrostatic recording and electrostatic printing. In addition, the present invention also relates to a method for manufacturing the toner; a toner container containing the toner; a developer including the toner; and an image forming method, an image forming apparatus and a process cartridge using the toner.
2. Discussion of the Background
Various electrophotographic image forming methods have been disclosed, for example, in U.S. Pat. No. 2,297,691.
In general, electrophotographic image forming methods typically include the following processes:                (1) an electrostatic latent image is formed on an image bearing member such as photoreceptors using one of various methods (latent image forming process);        (2) the electrostatic latent image is developed with a toner to form a visual image (toner image) on the image bearing member (developing process);        (3) the toner image is transferred to a receiving material such as papers optionally via an intermediate transfer medium (transferring process); and        (4) the toner image is fixed to the receiving material upon application of heat and/or pressure, resulting in formation of a copy (fixing process).        
Various methods have been proposed for the fixing method. Among the various fixing methods, a heat roller fixing method in which a heat roller is directly contacted to a toner image formed on a receiving material while applying pressure thereto has been broadly used because of having advantages such that the fixing device has good heat efficiency and the fixing device can be downsized.
However, the heat roller fixing method has a drawback in that a large power is needed to operate the fixing device. Therefore, various investigations have been made on heat roller fixing devices to reduce the power consumption thereof. For example, there is a proposal in that the thickness of the heat roller which is to be contacted with toner images is minimized as much as possible to improve the heat efficiency of the fixing operation and to shorten the temperature rising time which means the time needed for increasing the temperature of the heat roller in a waiting state to the predetermined fixing temperature.
However, such a fixing device has a drawback in that the heat capacity of the heat roller is decreased and thereby the difference in temperature between a portion (contact portion) of the heat roller contacting receiving material sheets and a portion thereof (non-contact portion) not contacting the receiving materials increases. In this case, if the fixing temperature is controlled while a sensor detects the contact portion, the temperature of the non-contact portion is excessively increased. Therefore, if a receiving material with large size is passed through the heat roller in this state, a hot offset problem in that the toner image thereon is adhered to the heat roller, and the toner image is re-transferred to an undesired portion of the receiving material or the following receiving material tends to occur at the portion of the toner image fixed by the excessively heated portion of the heated roller.
Recently, in order to save energy, investigations have been made on low temperature fixing techniques. In addition, investigations have been made on high speed image forming. Specifically, it has been attempted to develop toners having a low temperature fixability by using a low softening material such as resins and waxes for the toner. However, such low temperature fixable toners often causes a blocking problem in that toner particles are softened and aggregated due to the heat generated by the fixing device and the other image forming devices or when the toners are preserved at a high temperature. Namely, the toners have poor high temperature preservability. In addition, such toners tend to have a relatively narrow fixable temperature range.
In attempting to impart good combination of low temperature fixability and high temperature preservability (hereinafter simply referred to as preservability) to a toner, various investigations have been made. For example, a technique in that a polyester resin, which has a relatively good preservability and a relatively good low temperature fixability, is used as the binder resin of a toner is proposed. However, by using only this technique, it is impossible to impart good combination of preservability and low temperature fixability to the toner. This is because the two characteristics establish trade-off relationship.
In addition, published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 09-258480 discloses a toner including layered toner particles in which an outer portion of toner particles includes a resin having a glass transition temperature higher than that of the resin included in the inner portion of the toner particles. Such layered toner can be prepared by a method such as in-situ polymerization methods, interfacial polymerization methods, coacervation methods, spray drying methods, and phase-inversion emulsion methods (disclosed in JP-A 05-66600).
JP-As 2000-347455 and 2001-022117 have disclosed layered toners prepared by a phase-inversion emulsion method, in which a particulate material having a high glass transition temperature is fixed on toner particles. The toners have slightly improved preservability, but do not have a wide fixable temperature range. Namely, the toner does not have good combination of preservability and low temperature fixability.
Japanese Patent No. 2,794,770 (i.e., JP-A 02-287554) discloses a toner having a layered structure in which the outer portion of the toner particles are made of a resin having a molecular weight higher than that of a resin constituting the inner portion thereof. However, the low temperature fixability of the toner is not satisfactory because the toner particles are covered with a high molecular weight resin.
Recently it is very important to save energy. Requirements for next generation image forming apparatuses are described in the DSM (Demand-side Management) program of IEA (International Energy Agency). There are several requirements therein such that the warm-up time should not be greater than 10 seconds and the power consumption in a waiting state should be not greater than 10 to 30 watt (which changes depending on the copying speed) in copiers having a copy speed not less than 30 cpm (copies per minutes).
In attempting to fulfill the requirements, various toners have been developed. For example, toners which uses a polyester resin as a binder resin instead of styrene-acrylic copolymers which have been conventionally used have been disclosed in JP-As 60-90344, 64-15755, 02-82267, 03-229264, 03-41470 and 11-305486. This is because polyester resins have relatively good fixability and good preservability compared to styrene-acrylic copolymers. In addition, JP-A 62-63940 discloses a toner including a specific non-olefin crystal polymer as a binder resin. Further, Japanese Patent No. 2,931,899 discloses a toner including a crystalline polyester as a binder.
However, even such toners cannot fulfill the requirements described in the DSM program.
Methods for manufacturing toners are broadly classified into pulverization methods and suspension polymerization methods.
Pulverization methods typically include the following steps:    (1) mixing a binder resin, a colorant, a charge controlling agent, etc.;    (2) melting and uniformly kneading the mixture, followed by cooling;    (3) pulverizing the mixture; and    (4) classifying the pulverized mixture.
The pulverization methods have the following drawbacks:    (1) A pulverizer is necessary and therefore the manufacturing cost increases.    (2) The pulverized mixture typically has a broad particle diameter distribution. Therefore, when a toner having a relatively small average particle diameter and a narrow particle diameter distribution (for example, from 5 to 20 μm) is prepared to produce high quality images, the yield seriously deteriorates.    (3) It is difficult to uniformly disperse a colorant, a charge controlling agent, etc. in a binder resin. Therefore the toner is poor in fluidity, developability, durability and image qualities.
In suspension polymerization methods, toner constituents including a polymerizable material is suspended in a solvent, followed by polymerization of the polymerizable material, resulting in preparation of toner particles.
The toners prepared by such suspension polymerization methods do not have the drawbacks specific to the toners prepared by pulverization methods. However, the toners have the following drawbacks:    (1) Since the toners have a spherical form, toner particles remaining on image bearing members even after a toner image transfer process cannot be well removed with a cleaning blade. Therefore, a problem in that when an image having a large image area proportion is produced, a large amount of toner articles remain on the image bearing member, thereby causing background fouling in the following copy images. In addition, such residual toner particles contaminate the charging roller or the other elements contacting the image bearing member, and thereby a problem in that image qualities deteriorate occurs.
In addition, Japanese Patent No. 2,537,503 (i.e., JP-A 63-186253) discloses an emulsion polymerization method in which small particles are associated to prepare toner particles having irregular forms. However, the toner has a drawback in that a large amount of surfactant is included in the toner particles, and thereby the toner has poor charge properties (i.e., the toner has wide charge quantity distribution). Therefore, the resultant toner images have background fouling. In addition, the photoreceptor, charging roller and developing roller are contaminated, resulting in deterioration of images.
Recently, an emulsion-aggregation method in which a polymer dissolved in an organic solvent is dispersed in an aqueous medium is polymerized to prepare toner particles is disclosed in Japanese Patent No. 3,141,783 (i.e., JP-A 10-26842). It is described therein that by using the method, a toner having a core-shell structure can be produced. However, the shell serves only to prevent the pigments and waxes from being exposed to the outside. Namely, the surface conditions of the toner particles are not improved by this method. Therefore, the preservability and charge stability of the toner are not improved.
In the conventional suspension polymerization methods, emulsion polymerization methods and emulsion-aggregation methods mentioned above, styrene-acrylic resins are typically used as a binder resin. However, polyester resins, which have good fixability have hardly been used because it is hard to granulize polyester resins and to control the particle diameter, particle diameter distribution and toner particle form. Therefore, JP-A 09-34167 discloses a toner, which is prepared by changing the form of a toner including a polyester resin in an aqueous medium so as to be a spherical form. In addition, JP-A 11-149180 discloses a toner in which toner particles are prepared using an isocyanate.
However, these toners have drawbacks in that the productivity is low and low temperature fixability is not satisfactory.
Thus, there is no toner, which has good combination of high temperature preservability and low temperature fixability.